setExtension method
Sets the value of a non-repeated extension field to value.
Implementation
void setExtension(Extension extension, Object value) {
  if (PbFieldType.isRepeated(extension.type)) {
    throw ArgumentError(
      _fieldSet._setFieldFailedMessage(
        extension,
        value,
        'repeating field (use get + .add())',
      ),
    );
  }
  _fieldSet._ensureExtensions()._setFieldAndInfo(extension, value);
}